RNAi is the sequence-specific, post-transcriptional silencing of a gene's expression by double-stranded RNA. RNAi is mediated by 21 to 25 nucleotide, double-stranded RNA molecules referred to as small interfering RNAs (siRNAs) that are derived by enzymatic cleavage of long, double-stranded RNA in cells. siRNAs can also be synthesized chemically or enzymatically outside of cells and then delivered to cells (e.g., by transfection) (see, e.g., Fire et al., 1998, “Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans,” Nature, 391:806-11; Tuschl et al., 1999, “Targeted mRNA degradation by double-stranded RNA in vitro,” Genes Dev., 13:3191-7; Zamore et al., 2000, “RNAi: double-stranded RNA directs the ATP-dependent cleavage of mRNA at 21 to 23 nucleotide intervals,” Cell, 101:25-33; Elbashir et al., 2001, “Duplexes of 21-nucleotide RNAs mediate RNA interference in mammalian cell culture,” Nature, 411:494-498; and Elbashir et al., 2001, “RNA interference is mediated by 21- and 22-nucleotide RNAs,” Genes Dev., 15:188-200.
Double-stranded siRNAs mediate gene silencing by targeting for disruption or cleavage messenger RNAs (mRNAs) that contain the sequence of one strand of the siRNA. siRNAs introduced into mammalian cells by transfection mediate sequence-specific gene silencing, whereas long, double-stranded RNA induces sequence non-specific responses.